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Opportunistic Security for HTTP/2mnot@mnot.nethttps://www.mnot.net/Mozillamartin.thomson@gmail.comApplications and Real-Time
HTTPInternet-DraftThis document describes how http URIs can be accessed using Transport Layer Security (TLS) and
HTTP/2 to mitigate pervasive monitoring attacks. This mechanism not a replacement for https URIs;
it is vulnerable to active attacks.This document describes a use of HTTP Alternative Services to decouple the URI scheme
from the use and configuration of underlying encryption. It allows an http URI to be accessed
using HTTP/2 and Transport Layer Security (TLS) with Opportunistic
Security .This document describes a usage model whereby sites can serve http URIs over TLS, thereby
avoiding the problem of serving Mixed Content (described in )
while still providing protection against passive attacks.Opportunistic Security does not provide the same guarantees as using TLS with https URIs,
because it is vulnerable to active attacks, and does not change the security context of the
connection. Normally, users will not be able to tell that it is in use (i.e., there will be no
“lock icon”).The immediate goal is to make the use of HTTP more robust in the face of pervasive passive
monitoring .A secondary (but significant) goal is to provide for ease of implementation, deployment and
operation. This mechanism is expected to have a minimal impact upon performance, and require a
trivial administrative effort to configure.Preventing active attacks (such as a Man-in-the-Middle) is a non-goal for this specification.
Furthermore, this specification is not intended to replace or offer an alternative to https, since
https both prevents active attacks and invokes a more stringent security model in most clients.The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
“RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in
.An origin server that supports the resolution of http URIs can indicate support for this
specification by providing an alternative service advertisement for a protocol
identifier that uses TLS, such as h2. Such a protocol MUST include an explicit
indication of the scheme of the resource. This excludes HTTP/1.1; HTTP/1.1 clients are forbidden
from including the absolute form of a URI in requests to origin servers (see Section 5.3.1 of
).A client that receives such an advertisement MAY make future requests intended for the associated
origin to the identified service (as specified by ), provided that the
alternative service opts in as described in .A client that places the importance of protection against passive attacks over performance might
choose to withhold requests until an encrypted connection is available. However, if such a
connection cannot be successfully established, the client can resume its use of the cleartext
connection.A client can also explicitly probe for an alternative service advertisement by sending a request
that bears little or no sensitive information, such as one with the OPTIONS method. Likewise,
clients with existing alternative services information could make such a request before they
expire, in order minimize the delays that might be incurred.Client certificates are not meaningful for URLs with the http scheme, and therefore clients
creating new TLS connections to alternative services for the purposes of this specification MUST
NOT present them. A server that also provides https resources on the same port can request a
certificate during the TLS handshake, but it MUST NOT abort the handshake if the client does not
provide one.It is possible that the server might become confused about whether requests’ URLs have a http or
https scheme, for various reasons; see . To ensure that the alternative service has
opted into serving http URLs over TLS, clients are required to perform additional checks before
directing http requests to it.Clients MUST NOT send http requests over a secured connection, unless the chosen alternative
service presents a certificate that is valid for the origin as defined in . Using an
authenticated alternative service establishes “reasonable assurances” for the purposes of
. In addition to authenticating the server, the client MUST have obtained a valid
http-opportunistic response for an origin (as per ) using the authenticated
connection. An exception to the latter restriction is made for requests for the
“http-opportunistic” well-known URI.For example, assuming the following request is made over a TLS connection that is successfully
authenticated for those origins, the following request/response pair would allow requests for the
origins “http://www.example.com” or “http://example.com” to be sent using a secured connection:Though this document describes multiple origins, this is only for operational convenience. Only
a request made to an origin (over an authenticated connection) can be used to acquire this
resource for that origin. Thus in the example, the request to http://example.com cannot be
assumed to also provide an http-opportunistic response for http://www.example.com.Clients MUST NOT send http requests and https requests on the same connection. Similarly,
clients MUST NOT send http requests for multiple origins on the same connection.This specification defines the “http-opportunistic” well-known URI . A client is said to
have a valid http-opportunistic response for a given origin when:The client has requested the well-known URI from the origin over an authenticated connection
and a 200 (OK) response was provided, andThat response is fresh (potentially through revalidation ), andThat response has the media type “application/json”, andThat response’s payload, when parsed as JSON , contains an array as the root, andThe array contains a string that is a case-insensitive character-for-character match
for the origin in question, serialised into Unicode as per Section 6.1 of .A client MAY treat an “http-opportunistic” resource as invalid if values it contains are not
strings.This document does not define semantics for “http-opportunistic” resources on an https origin,
nor does it define semantics if the resource includes https origins.Allowing clients to cache the http-opportunistic resource means that all alternative services
need to be able to respond to requests for http resources. A client is permitted to use an
alternative service without acquiring the http-opportunistic resource from that service.A client MUST NOT use any cached copies of an http-opportunistic resource that was acquired (or
revalidated) over an unauthenticated connection. To avoid potential errors, a client can
request or revalidate the http-opportunistic resource before using any connection to an
alternative service.Clients that use cached http-opportunistic responses MUST ensure that their cache is cleared of
any responses that were acquired over an unauthenticated connection. Revalidating an
unauthenticated response using an authenticated connection does not ensure the integrity of the
response.This specification registers a Well-Known URI :URI Suffix: http-opportunisticChange Controller: IETFSpecification Document(s): of [this specification]Related Information:User Agents MUST NOT provide any special security indicators when an http resource is acquired
using TLS. In particular, indicators that might suggest the same level of security as https MUST
NOT be used (e.g., a “lock device”).A downgrade attack against the negotiation for TLS is possible.For example, because the Alt-Svc header field likely appears in an unauthenticated
and unencrypted channel, it is subject to downgrade by network attackers. In its simplest form, an
attacker that wants the connection to remain in the clear need only strip the Alt-Svc header
field from responses.Cached alternative services can be used to track clients over time; e.g., using a user-specific
hostname. Clearing the cache reduces the ability of servers to track clients; therefore clients
MUST clear cached alternative service information when clearing other origin-based state (i.e.,
cookies).HTTP implementations and applications sometimes use ambient signals to determine if a request is
for an https resource; for example, they might look for TLS on the stack, or a server port number
of 443.This might be due to expected limitations in the protocol (the most common HTTP/1.1 request form
does not carry an explicit indication of the URI scheme and the resource might have been developed
assuming HTTP/1.1), or it may be because how the server and application are implemented (often,
they are two separate entities, with a variety of possible interfaces between them).Any security decisions based upon this information could be misled by the deployment of this
specification, because it violates the assumption that the use of TLS (or port 443) means that the
client is accessing a HTTPS URI, and operating in the security context implied by HTTPS.Therefore, server implementers and administrators need to carefully examine the use of such signals
before deploying this specification.This specification requires that a server send both an Alternative Service advertisement and host
content in a well-known location to send HTTP requests over TLS. Servers SHOULD take suitable
measures to ensure that the content of the well-known resource remains under their control.
Likewise, because the Alt-Svc header field is used to describe policies across an entire origin,
servers SHOULD NOT permit user content to set or modify the value of this header.Key words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.HTTP Over TLSThis memo describes how to use Transport Layer Security (TLS) to secure Hypertext Transfer Protocol (HTTP) connections over the Internet. This memo provides information for the Internet community.The Transport Layer Security (TLS) Protocol Version 1.2This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK]Defining Well-Known Uniform Resource Identifiers (URIs)This memo defines a path prefix for "well-known locations", "/.well-known/", in selected Uniform Resource Identifier (URI) schemes. [STANDARDS-TRACK]The Web Origin ConceptThis document defines the concept of an "origin", which is often used as the scope of authority or privilege by user agents. Typically, user agents isolate content retrieved from different origins to prevent malicious web site operators from interfering with the operation of benign web sites. In addition to outlining the principles that underlie the concept of origin, this document details how to determine the origin of a URI and how to serialize an origin into a string. It also defines an HTTP header field, named "Origin", that indicates which origins are associated with an HTTP request. [STANDARDS-TRACK]The JavaScript Object Notation (JSON) Data Interchange FormatJavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data.This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance.Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and RoutingThe Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document provides an overview of HTTP architecture and its associated terminology, defines the "http" and "https" Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes related security concerns for implementations.Hypertext Transfer Protocol (HTTP/1.1): Conditional RequestsThe Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP/1.1 conditional requests, including metadata header fields for indicating state changes, request header fields for making preconditions on such state, and rules for constructing the responses to a conditional request when one or more preconditions evaluate to false.Hypertext Transfer Protocol (HTTP/1.1): CachingThe Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages.Hypertext Transfer Protocol Version 2 (HTTP/2)This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol (HTTP), referred to as HTTP version 2 (HTTP/2). HTTP/2 enables a more efficient use of network resources and a reduced perception of latency by introducing header field compression and allowing multiple concurrent exchanges on the same connection. It also introduces unsolicited push of representations from servers to clients.This specification is an alternative to, but does not obsolete, the HTTP/1.1 message syntax. HTTP's existing semantics remain unchanged.HTTP Alternative ServicesThis document specifies "Alternative Services" for HTTP, which allow an origin's resources to be authoritatively available at a separate network location, possibly accessed with a different protocol configuration.Pervasive Monitoring Is an AttackPervasive monitoring is a technical attack that should be mitigated in the design of IETF protocols, where possible.Opportunistic Security: Some Protection Most of the TimeThis document defines the concept "Opportunistic Security" in the context of communications protocols. Protocol designs based on Opportunistic Security use encryption even when authentication is not available, and use authentication when possible, thereby removing barriers to the widespread use of encryption on the Internet.Public Key Pinning Extension for HTTPThis document defines a new HTTP header that allows web host operators to instruct user agents to remember ("pin") the hosts' cryptographic identities over a period of time. During that time, user agents (UAs) will require that the host presents a certificate chain including at least one Subject Public Key Info structure whose fingerprint matches one of the pinned fingerprints for that host. By effectively reducing the number of trusted authorities who can authenticate the domain during the lifetime of the pin, pinning may reduce the incidence of man-in-the-middle attacks due to compromised Certification Authorities.Mixed ContentMike Bishop contributed significant text to this document.Thanks to Patrick McManus, Stefan Eissing, Eliot Lear, Stephen Farrell, Guy Podjarny, Stephen Ludin,
Erik Nygren, Paul Hoffman, Adam Langley, Eric Rescorla, Julian Reschke, Kari Hurtta, and Richard
Barnes for their feedback and suggestions.